The neurointermediate lobe (NIL) of the rat adenohypophysis plays an essential role in regulating the dynamic release of prolactin (PRL) from the anterior pituitary (AP). This is evidenced by reports that surgical removal of the NIL blocks the acute release of PRL induced by two physiologically relevant stimuli-sucking and estrogen treatment. The NIL communicates with the AP via the short portal vessels, and chemical signals which convert the effects of evocative stimuli to changes in PRL cell function are probably delivered by this route. We have recently obtained compelling evidence that the NIL contributes two relevant activities to these processes. The first of these appears to be released by the suckling stimulus and renders mammotrophs more responsive to PRL- releasing agents. The other is induced by estrogen and acutely recruits additional PRL-releasing cells into the secretory population. Interestingly, both the responsiveness and recruitment activities can be mimicked in vitro by the di-acetylated variant of alpha-melanocyte stimulating hormone (MSH) secreted by the rat NIL. Accordingly, our overall goal is to elucidate how the NIL, through the mediation of this peptide, plays a requisite role in the dynamic release of PRL. In pursuit of this objective, we will test the hypotheses that: 1) acetylated alpha-MSH contribution to suckling-induced PRL release; and 2) acetylated alpha-MSH released from the NIL also mediates estrogen- induced PRL release by recruitment of additional mammotrophs. We plan to take a multificated approach to these problems. Reverse hemolytic plaque assays will be employed to analyze hormone secretin at the single cell level. Molecular strategies will provide complementary information on gene expression. In addition, several in vivo studies are planned to confirm the biological relevance of our in vitro findings. Thus, by integrating information derived from studies at multiple levels of organizational complexity-cellular, molecular, and organismal-we intend to gain a comprehensive understanding about the neuroendocrine mechanisms that govern the dynamic release of PRL. Such information is essential for identifying the causes of, and developing treatments for, pathologies of PRL release in humans.